This application claims the benefit of Korean Application No. 2000-27749, filed May 23, 2000, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical pickup capable of detecting a tracking error signal with reduced push-pull offset when an objective lens is shifted in a radial direction of an optical disk, and/or a reproduction signal including less crosstalk from adjacent tracks.
2. Description of the Related Art
The capacity of optical recording media for information recording and reproduction is dependent upon the size of a light spot focused on its information recording surface, and its track pitch whose width is determined in consideration of the light spot size. That is, the smaller the light spot and the track pitch, the greater the recording capacity.
As can be inferred from equation (1) below, the size of the light spot is proportional to the wavelength (xcex) of the light, and is inversely proportional to the numerical aperture (NA) of an objective lens that focuses incident light to form the light spot on an information recording surface.
size of the light spotxe2x88x9dxcex/NAxe2x80x83xe2x80x83(1) 
On the other hand, the track pitch (tp) of an optical recording medium is determined by equation (2) so as to minimize track-to-track interference with respect to a reproduction signal.                               t          p                 greater than                   0.6          xc3x97                      λ            NA                                              (        2        )            
For example, for a compact disk (CD), which uses a light source having a wavelength of 780 nm and an objective lens having an NA of 0.45, the track pitch of the CD is determined at 1.6 xcexcm, which is greater than the result of 1.04 xcexcm found using equation (2) with a xcex=780 nm and an NA=0.45. For a digital versatile disk (DVD), which uses a light source having a wavelength of 650 nm and an objective lens having an NA of 0.6, the track pitch of the DVD is determined at 0.74 xcexcm (equivalent to a recording density of 4.7 gigabytes), which is greater than the result of 0.65 xcexcm found using equation (2) using a xcex=650 nm and an NA=0.6.
Standard CDs and DVDs are designed to have a track pitch that satisfies the condition of equation (2), and thus considerable degradation of a reproduction signal caused by adjacent tracks does not occur during recording/reproduction operation.
Meanwhile, for future generation DVDs, so-called xe2x80x9chigh-definition DVDsxe2x80x9d (HD-DVDs), which have been developed to increase the recording density up to 15 gigabytes or more, equation (2) above cannot be used to determine a standard track pitch of the HD-DVDs. For example, given that an optical pickup using a 400 nm light source and an objective lens having an NA of 0.6 is used with HD-DVDs, the track pitch calculated using the right side of equation (2) above, is 0.4 xcexcm. However, the track pitch of 0.4 xcexcm is too wide to achieve the high recording capability of 15 GB or more. Thus, a standard track pitch of HD-DVDs should be 0.4 xcexcm or less.
When reproducing information from HD-DVDs currently under development, there is also a problem of crosstalk between adjacent tracks. Thus, there is a need to reduce degradation of a reproduction signal caused by interference between adjacent tracks due to the narrow track pitch.
At the same time, for recordable optical disks, a push-pull signal is detected as a tracking error signal. A drawback of the push-pull signal is that a large offset occurs when the objective lens is shifted in the radial direction of an optical disk. In order to reduce the push-pull offset, a conventional technique uses a grating such that a main light spot and first and second sub-light spots are focused on the main track and adjacent tracks of an optical recording medium, thereby detecting a tracking error signal using a differential push-pull technique.
A conventional optical pickup using such a grating is shown in FIG. 1. Light emitted from a light source 1 is diffracted and split into a 0th-order diffracted beam 1a and xc2x11st-order diffracted beams 1b, 1c by a grating 2. The split beams 1a, 1b, 1c are reflected by a beam splitter 3 and a mirror 4, collimated by a collimating lens 6, and focused by an objective lens 5. As a result, light spots of beams 1a, 1b, 1c are formed on an optical disk 10 as shown in FIG. 2. The main beam 1a, which is the 0th-order diffracted beam, is focused on the target track 12, and the first and second sub-beams 1b and 1c are focused to be displaced from the main beam 1a by xc2x1xc2xd the track pitch in the radial direction of the optical disk 10, leading and following the main beam 1a. 
After having been reflected from the optical disk 10, the main beam 1a and the first and second sub-beams 1b and 1c pass the objective lens 5 and the beam splitter 3, and are then received by a photodetection unit 8 through a sensing lens 7 to condense incident light on the first, second and third photodetectors 8a, 8b and 8c. The photodetection unit 8, as shown FIG. 2, includes a first photodetector 8a to receive the main beam 1a, and the second and third photodetectors 8b and 8c to receive the first and second sub-beams 1b and 1c, respectively. Each of the first, second and third photodetectors 8a, 8b and 8c performs photoelectric conversion, and consists of two split plates arranged in the radial direction of the optical disk.
A tracking error signal detection unit 20 detects a tracking error signal using a differential push-pull technique, which is used in the conventional optical pickup. The tracking error signal detection unit 20 includes first, second, third and fourth differential parts 21, 23, 25 and 29, and first and second amplifiers 27 and 28, and detects a tracking error signal, which contains no push-pull offset, by the conventional differential push-pull technique.
In particular, the first through third differential parts 21, 23, 25 receive the electrical signals from the first through third photodetectors 8a, 8b and 8c, and output first through third push-pull signals, respectively. A first amplifier 27 amplifies the third push-pull signal from the third differential part 25 with a predetermined gain factor G1, and a second amplifier 28 amplifies the sum of the signal output from the first amplifier 27 and the second push-pull signal from the second differential part 23 with a predetermined gain factor G2. The fourth differential part 29 subtracts the signal output from the second amplifier 28 from the first push-pull signal resulting from the main beam 1a, which is output from the first differential part 21, and outputs a tracking error signal. The gain factors G1 and G2 for the first and second amplifiers 27 and 28 are determined based on the intensities of the main beam 1a and the first and second sub-beams 1b and 1c. The fourth differential part 29 outputs a tracking error signal without push-pull offset and thus the tracking error signal detected in the conventional optical pickup includes no push-pull offset even when the objective lens is shifted in the radial direction of the optical disk.
However, the conventional optical pickup detects the tracking error signal by splitting light emitted from the light source 1 into three beams using the grating 2, so that the light efficiency of the main beam 1 is insufficient to record information on the optical disk. In addition, during a recording operation, information signals recorded on adjacent tracks are possibly erased by the first and second sub-beams 1b and 1c. 
To solve the above and other problems, it is an object of the present invention to provide an optical pickup suitable for high-density optical recording media having narrow track pitch, in which a single light spot is focused on only a main track of an optical recording medium during recording so that information signals recorded on adjacent tracks are not erased, and where a signal can be detected with reduced offset when an objective lens is shifted in the radial direction of the optical recording medium such that a reproduction signal having less crosstalk from adjacent tracks can be detected.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, an optical pickup according to an embodiment of the present invention includes a light source, an optical path changer to alter the traveling path of incident light, an objective lens disposed on the optical path between the optical path changer and an optical recording medium, the objective lens to focus incident light, a light detection unit to detect light incident through the objective lens and the optical path changer after having been reflected from the optical recording medium, and a signal processor having a tracking error signal detection portion to detect a tracking error signal from detection signals output from the light detection unit, wherein light emitted from the light source is focused as a single light spot on the optical recording medium, the light detection unit divides light reflected from the optical recording medium into first through fourth light beam portions in the radial direction of the optical recording medium, and detects the first through fourth light beam portions, and the tracking error signal detection portion amplifies or attenuates detection signals from the first and fourth light beam portions, which are outwards of the second and third light beam portions, or from the second and third light beam portions, and detects a push-pull signal using the gain controlled detection signals and the remaining detection signals.
According to an aspect of the present invention, the tracking error signal detection portion comprises a first differential part to subtract the detection signals from the first and third light beam portions, a second differential part to subtract the detection signals from the second and third light beam portions, an adder to sum the signals from the first and second differential parts, and a gain controller connected between the first or second differential part and an input end of the first adder, the gain controller to amplify or attenuate the signal from the first or second differential part with a predetermined gain factor, and to output the product to the first adder so that the first adder outputs a tracking error signal with reduced push-pull offset.
According to another aspect of the present invention, the tracking error signal detection portion detects a tracking error signal expressed as
tracking error signal=(axe2x88x92d)+xcex1(bxe2x88x92c) 
where a, b, c and d represent the detection signals from the first through fourth light beam portions, respectively, and xcex1 is the gain factor of the gain controller.
According to still another aspect of the present invention, the signal processor further comprises a reproduction signal detection portion including a gain controller to amplify or attenuate a first sum signal of the detection signals from the first and fourth light beam portions, or a second sum signal of the detection signals from the second and third light beam portions, with a predetermined gain factor, and an adder having a first input end that receives the signal from the gain controller and a second end that receives one of the first and second sum signals, which does not pass through the gain controller, the adder to sum the received signals and to output the sum of the received signals as a reproduction signal containing less crosstalk from adjacent tracks.
According to yet another aspect of the present invention, the light detection unit is a photodetector having first through fourth light receiving portions arranged in a radial direction of the optical recording medium, the first through fourth light receiving portions to receive the first through fourth light beam portions, respectively, and to independently convert the first through fourth light beam portions to electrical signals.
According to a further aspect of the present invention, the light detection unit comprises an optical member to selectively diffract light incident after having been reflected from the optical recording medium so as to divide the incident light into the first through fourth light beam portions in the radial direction of the optical recording medium, and first through fourth photodetectors to receive the first through fourth light beam portions from the optical member, and to convert the received first through fourth light receiving portions to corresponding electrical signals.